In a battery consisting of a plurality of cells, the self-discharge of the individual cells is a problem, since the self-discharge current can vary heavily from cell to cell. The trickle charge supplied to the battery must be at least as large as the greatest self-discharge current to avoid some particular cell from being discharged after a time. If the same current goes through all the cells, cells with low self-discharge will get a charging current which is too large, resulting in overvoltage in these cells. An overvoltage has, inter alia, the following drawbacks.
(1) It results in troublesome variation in the voltage of the individual cells. PA0 (2) It negatively affects the lifetime of a cell. PA0 (3) It results in the generation of gas in the cell. PA0 (4) The risk of injurious excess pressure is increased with respect to hermetically sealed cells. PA0 (5) Water consumtion increases in open cells.
A known techique for tailoring the trickle current to the requirement of the individual cell is to connect a resistor in parallel with each cell, this resistor having a resistance which is so low that the current through it is considerably larger than the largest self-discharge current. Each cell will thus be charged up to the same pole (terminal) voltage. A disadvantage with this method is that if there is an interruption in the charging current, the battery will be discharged across these resistors with troublesome rapidity. Another disadvantage is that the energy consumption in the resistors is very high during normal operation. Another known technique is to use series-connected diodes instead of resistors. The disadvantage with battery discharge when the charge current is interrupted is thus elimintated. A problem with this technique is that variations in the characteristic of the diodes are not compensated, and thus only a modest regulating effect is obtained.